The nature of the enzyme and template changes which give rise to alterations of rRNA transcription rates in vivo are not well established. The work proposed here will address these questions by use of an investigational protocol in which a defined response to a pharmacological agent is produced in rRNA transcription in vivo as determined in isolated nuclei and the molecular mechanisms underlying that response are characterized by enzymological and transcriptional analyses of the transcribing enzyme, RNA polymerase I, and the template, nucleolar chromatin. Analysis of transcriptional products in vitro will be use of G/U incorporation ratios, hybridization to 28S rRNA: cDNA, and T1 fingerprinting. The ability of the transcripts to combine with ribosomal proteins will also be investigated. Characterization of the enzymological properties of the RNA polymerases I extracted from the rat liver nuclei after the various in vivo treatments described will be by analysis of initiation and reaction kinetics on defined synthetic templates, and by gel electrophoretic investigation (both one and two dimensional) of the molecular architecture of the enzymes. RNA polymerases I from the variously treated animals will be further analyzed in terms of their ability to specifically transcribe (as defined above) nucleolar chromatin from controls and treated animals, As a part of the enzyme studies, the initial characterization of systems which act to modify the enzyme in response to the agents used will be undertaken. Analysis of the template properties of nucleolar chromatin from the variously treated animals will be in terms of its ability to serve as a template for specific transcription of rRNA sequences by purified RNA polymerase I. Since the control of rRNA synthesis is of considerable importance, and may be rate limiting, for the control of protein biosynthesis the studies proposed are seen as significant. Earlier studies have not combined careful analysis of transcription products, enzymology and structure of RNA polymerase, I and template activity with experimentally manipulatable changes in rRNA transcription. The studies proposed here will serve, therefore, to establish the relationship of in vivo alterations in rRNA transcription, modification of the transcriptional apparatus, and regulation of such modification.